Blast, also called rotten neck, is one of the most destructive
diseases of Missouri rice. Blast does not develop every year but
is very destructive when it occurs.

Rice blast can be controlled by a combination of preventative
measures. Whenever possible, consult an expert in plant disease
diagnosis; check with the University of Missouri-Delta Center,
(573) 379-5431, your local University Extension Center, or the
University of Missouri Plant Diagnostic Center in Columbia, MO,
(573) 882-3019.

Symptoms

Blast symptoms can occur on leaves, leaf collars, nodes and
panicles. Leaf spots are typically diamond shaped, with gray-
white centers and brown to red-brown margins
(Figure 1). Fully
developed leaf lesions are approximately 0.4 to 0.7 inch long and
0.1 to 0.2 inch wide. Both the shape and color vary depending on
the environment, age of the lesion and rice variety.

The most serious damage occurs when the disease develops on
the node just below the panicle. The "neck" often breaks at the
diseased node. This stage of the disease is referred to as
"rotten neck." Disease in the node prevents the flow of water
and nutrients to the kernels and they will stop filling. Heads
of plants damaged in this way may be completely blank to near
normal, depending on the stage of head development when infection
occurs. The poorly developed grain usually breaks up badly in
milling, reducing quality.

Causal agent

This disease is caused by a fungus known as Pyricularia
grisea, which overwinters in rice seeds and infected rice stubble
in the deep south. We do not know if it survives overwinter in
Missouri. The fungus spores, which are the reproductive
structures, can spread from these two sources to rice plants
during the next growing season and initiate new infections.
Spores from these new infections can spread by wind to other rice
plants over great distances.

There are several races of Pyricularia grisea. To date, four
major races have been found in Arkansas IG1, IH1, IC17 and
IB49. Since 1993, Dr. Fleet Lee in Arkansas has also collected a
new race, IE-1k, that can attack the blast-resistant varieties
Katy and Kaybonnet. So far, this new race has been of minor
importance but may become a bigger problem if large acreage of
Kaybonnet is planted in coming years. Race IB49 was found in
Missouri in 1997.

Control

Use preventive measures

Incorporate or roll the rice stubble soon after
harvest to promote early decomposition.

Grow rice in open fields free of tree lines
particularly on east and south sides.

Grow rice in fields where flood levels are easily
maintained. Damage from blast can be reduced by
keeping soil flooded 2-4 inches deep from the time
rice plants are 6 to 8 inches tall until draining
for harvest. Draining for straighthead is
incompatible with the flooding required for blast
control, so avoid fields with a history of
straighthead and varieties susceptible to
straighthead or plant blast resistant varieties in
these fields (see Table 1).

Seed over a range of time to spread the heading
dates. However, avoid planting late because blast
will be more severe.

Seed to a stand of 15 to 20 plants per square
foot.

Avoid excessive nitrogen application rates and
apply no more than 30 pounds per acre of nitrogen
per application at midseason. In fields with a
history of blast, always split applications.

Use fungicides

Scout fields for blast symptoms from the seedling through
heading stages (see Scouting). If symptoms are found, prepare to
use fungicides at the late boot stage and again when 80-90
percent of plants are headed (click here for list).

Apply fungicides during the time frame predicted by the DD50
program, a rice growth prediction program provided through the
local University Extension centers; this period is about 5 to 7
days before heading (late boot stage). Fungicides are especially
needed if blast symptoms have been observed in the field and the
variety is very susceptible. Fungicides should be applied a
second time at 80-90 percent head exsertion (heading). In
uniform stands, 90 percent heading will occur in 4 to 5 days
after the first heads are visible.

The decision to treat is more easily made when one or more of
the following factors exist:

A susceptible variety is grown in the field.

The crop has excessive growth and a dense canopy.

Leaf symptoms have been found in the field.

Disease is present in southern parts of the field.

Cool, rainy, or cloudy weather with high humidity
and heavy dews is predicted during heading.

The development of this disease is difficult to predict, and
fungicide treatments are expensive. Therefore, you should treat
on the basis of the above factors or automatically treat the
field with a fungicide if you are unwilling to risk disease
damage.

Benomyl can be used to control blast. The proper rates are
1.0 to 1.5 pounds per acre. The timing of benomyl applications
at the full boot and head exsertion growth stages was derived
from years of fungicide research. Yield responses were the
greatest when benomyl was applied at those two stages rather than
others.

A surfactant may enhance fungicidal control of blast as shown
in some Arkansas tests. The additions of a surfactant may be
useful if cost is not excessive.

Scouting

Leaf symptoms will appear most readily on plants at the edges of
fields, on levees, in areas of the fields that are shaded in the
morning, or in areas that received excessive nitrogen. Symptoms
are usually worse on drought-stressed rice.

If you are uncertain about diagnosing blast symptoms, send a
sample to your local University of Missouri Extension center for
identification. Symptoms present early in the season should warn
you that the disease may occur in midseason.

You should continue to scout for blast near the heading stage
and watch carefully for flagleaf collar symptoms on early-planted
susceptible varieties. Also, devote time to determine the stage
of rice development to see if the DD50 predicted time frame for
fungicide treatment for blast is correct. Symptoms appear 4 to 6
days after infection, so rice heads may be infected without
symptoms appearing.

Often-asked questions

Q: Do blast spores overwinter in Missouri?
A: We don't know, but spores were found in Arkansas during
the winter of 1987-1988 in standing stubble. Preliminary
research indicates other inoculum sources exist including
seed.

Q: Can my rice field get blast from infected seed?
A: The fungus has been isolated at low levels from seed.
Seed may carry blast spores that could cause early
seedling infection and symptoms.

Q: Will fungicide seed treatment prevent blast?
A: Seed treatment in general is good insurance against
seedling diseases and is likely to stop any seed
transmission of blast. It will not prevent infection from
wind-borne spores. Not all seed treatment fungicides are
effective. Use a broad-spectrum material.

Q: Will burning the straw from an infected field help?
A: Burning of stubble may reduce trash borne spores, but the
disease can still spread to your field by wind-borne
spores.

Q: Will rolling the straw soon after harvest be as effective
as burning?
A: It could be more effective because of the difficulty in
getting a thorough burn, and the crop residue can have a
beneficial effect on the rotated crop. It may be more
advantageous to burn when rice must follow rice.

Q: Should I use a fungicide automatically for blast?
A: Fungicides are expensive and should only be used when
conditions warrant.

Q: When is it too late to apply a fungicide?
A: Fungicides should not be used after grain is in the milk
stage and panicles are turning over, or approximately 10
days after full heading.

Q: Should I treat with a fungicide if severe blast is
discovered on tillering rice?
A: Rather than treat tillering rice, you should fertilize
with nitrogen as needed, maintain a uniform flood of 2-4
inches depth and scout fields closely to determine disease
progress. Treatment with a fungicide may reduce the
inoculum but is probably not cost effective. Head
protection will be more economical.

Q: What should be done to a field that has both sheath blight
and blast?
A: Fungicides should be applied to control the disease to
which the variety is the most susceptible. A fungicide
that is effective on both diseases is best. Use the most
effective fungicide for sheath blight control if sheath
blight is the only disease present. However, if blast is
present or anticipated, use the most effective fungicide
for blast control at the second application.

MP0645, Rice Blast Control (XPLOR only)

* Issued in furtherance of Cooperative Extension Work Acts of May 8 and
June 30, 1914, in cooperation with the United States Department of
Agriculture. Ronald J. Turner, Director, Cooperative Extension Service,
University of Missouri and Lincoln University, Columbia, Missouri 65211.
* University Extension does not discriminate on the basis of race, color,
national origin, sex, religion, age, disability or status as a Vietnam-era
veteran in employment or programs. * If you have special needs as addressed
by the Americans with Disabilities Act and need this publication in an
alternative format, write ADA Officer, Extension and Agricultural
Information, 1-98 Agriculture Building, Columbia, MO 65211, or call (573)
882-8237. Reasonable efforts will be made to accommodate your special
needs.